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An equation is a mathematical statement, in symbols, that two things are the same (or equivalent). Equations are written with an equal sign. Equations are often used to state the equality of two expressions containing one or more variables.

An expression is a combination of numbers, operators, grouping symbols (such as brackets and parentheses) and/or free variables and bound variables arranged in a meaningful way which can be evaluated. Bound variables are assigned values within the expression (they are for internal use) while free variables can take on values from outside the expression.

A rational number is a number which can be expressed as a ratio of two integers. Non-integer rational numbers (commonly called fractions) are usually written as the vulgar fraction a / b , where b is not zero. a is called the numerator, and b the denominator.

Solving an equation means finding a value for the variable that makes the equation true.

How many legs in total do two chickens, two ducks,and two sheep have?